Optical switching unit, particularly for switching to standby components in optical transmission systems

ABSTRACT

An optical switching unit is disclosed which permits switching from faulty components to standby components in optical transmission systems. A component in this connection may be a transmit laser or an optical fiber, for example. The optical switching unit (OSE) according to the invention contains an n×m optical space switch (ORS) whose ports (P 1 A . . . PNA; P 1 B . . . PMB) are monitored by photodiodes (M 1 A, M 1 B). The photodiodes provide a control unit (SE) with information as to whether and, if so, what signals are arriving at the ports. The control unit (SE) controls the position of the optical switch (ORS) in accordance with program instructions. The switching unit controls itself, i.e., incorporation of a higher-level control center is not necessary. It thus allows very short switching times and has many applications; it is particularly suitable for use in modular optical transmission systems.

[0001] This invention relates to an optical switching unit, particularlyfor switching from faulty transmission facilities to standbytransmission facilities in optical transmission systems. Such atransmission facility may be a transmit laser, a receiver photodiode, oran optical fiber, for example.

[0002] In optical transmission systems, optical fibers interconnect atransmitter and a receiver. Because of the wide transmission bandwidthsof optical fibers, a single such fiber generally suffices to transmitthe usual amounts of data. To be able to continue to transmit data ifthe optical fiber should be damaged, one or more spare fibers are laidparallel to that optical fiber. If any damage to the optical fiber isdetected, a control unit will cause the transmitter and receiver toswitch to a spare fiber.

[0003] To increase the reliability of optical transmission systems, astandby laser and/or a standby photodiode, which are used in case offailure of the main device, are frequently incorporated into the system.Since communication is interrupted while changeover to a standbytransmission facility is effected, the dead time of the transmissionsystem, i.e., the time required for the changeover, should be as shortas possible.

[0004] European Patent Application EP-A1-519 712 discloses an opticalbus network in which the two end nodes of the bus are connected byoptical fibers to an additional monitoring node (“terminal node”). Themonitoring node, which is thus connected directly between the two endnodes, establishes a direct connection between the two end nodes withthe aid of a switch if there is a discontinuity in the bus. As a result,the nodes in the immediate vicinity of the discontinuity become the newend nodes of the bus. To be able to detect discontinuities (e.g., abreak in a fiber), the monitoring node includes optical-to-electricaltransducers with which it monitors the signals from the end nodes.

[0005] offenlegungsschrift DE-A1-44 33 031 discloses a method ofswitching to one or more spare lines in unidirectional or bidirectionaloptical transmission systems (see FIG. 6). If the receiver portion of atransceiver unit SE6A at one end of a bidirectional optical transmissionsystem does not receive the data stream transmitted over an opticalfiber FA6 or receives the data stream only in greatly disturbed form, itwill communicate this to a control unit ST6A associated with this end.The control unit causes the transmitter and receiver at this end to beswitched to a spare line FAE6. The receiver portion of a transceiverunit SE6B at the other end of the transmission link then determines thatno signal is now present on the originally used optical fiber FA6. Inresponse to this, the control unit ST6B initiates a changeover to thespare line FAE6 at that end of the transmission link as well.

[0006] Under certain circumstances, for example in the case of very longtransmission links, the probability that not only the optical fiberactually intended for transmission but also the spare fiber will bedamaged is relatively high. In that case, the data transmission willcollapse. To prevent this, more than one spare fiber may be laid, as isknown-from the above-cited DE-A1-44 33 031. Particularly in the case oflong transmission links, however, the additional optical fibers to belaid represent a considerable cost factor.

[0007] It is therefore an object of the invention to provide an opticalswitching unit with which changeover can be effected from faultytransmission facilities to standby transmission facilities in opticaltransmission systems. The optical switching unit is to be simple inconstruction and universally applicable. In particular, the opticalswitching unit is to permit the construction of an optical transmissionsystem which allows reliable data transmission even in the event offrequent damage to optical fibers but requires as few spare opticalfiber as possible.

[0008] The invention attains the object with the aid of the featuresrecited in claim 1. One, a number, or all of the ports on both sides ofan n×m optical space switch are monitored by photodiodes which areconnected to a control unit incorporated into the optical switchingunit. The control unit controls the position of the optical space switchin accordance with changes in the signals applied at the ports.Depending on how the control unit is programmed, the optical switchingunit according to the invention can be used to advantage for differentfunctions.

[0009] In a first embodiment, the optical switching unit is used as aswitch in an optical transmission link. The optical link consists of afirst optical fiber and a parallel, second optical fiber as a sparefiber. Optical switching units according to the invention are integratedinto the link at given intervals. If the first optical fiber is broken,the optical switching units adjacent to the break will detect this andswitch to the existing spare fiber. Since the switching processes areinitiated and controlled in the transmission link itself and notcentrally at the transmitter end, the dead time of the transmissionsystem, i.e., the time in which no data transmission takes place, isvery short.

[0010] The division of the transmission link into several sectionsensures reliable data transmission even if both optical fibers have oneor even more breaks, since, according to the invention, only therespective faulty section is bypassed by the spare fiber. Anotheradvantage is that the positions of the optical switches can also becontrolled from the transmitter directly via the optical fibers if theoptical switching unit is programmed in such a way as to detectparticular patterns in the data stream and initiate correspondingswitching processes in response thereto.

[0011] In a second embodiment, the optical switching unit is used in anarrangement of transmit lasers to switch to a standby laser if one ofthe transmit lasers fails. In that case, too, the transmissioncapability of the system can be restored in the shortest time.

[0012] In a third embodiment, the optical switching unit is used as anoptical selector switch. In packet-switched transmission systems, theoptical switching unit is capable of detecting the address field in thepacket header and of establishing connections in accordance with thisinformation. Thus, the switching unit according to the invention can beused to advantage in future optical ATM exchanges, for example.

[0013] Further advantageous features of the invention are defined in thesubclaims.

[0014] The invention will become more apparent from the followingdescription of embodiments when taken in conjunction with theaccompanying drawings, in which:

[0015]FIG. 1 is a schematic block diagram of an optical switching unitas claimed in claim 1;

[0016]FIG. 2 is a schematic block diagram of an optical switching unitas claimed in claim 3;

[0017]FIG. 3 is a schematic block diagram of an optical switching unitas claimed in claim 2;

[0018]FIG. 4a is a schematic block diagram of a circuit for switching toa spare fiber in a bidirectional optical transmission system, opticalfiber unbroken;

[0019]FIG. 4b is a schematic block diagram of a circuit for switching toa spare fiber in a bidirectional optical transmission system, opticalfiber broken;

[0020]FIG. 5 is a schematic block diagram of a circuit for switching toa spare fiber in a unidirectional optical transmission system, opticalfiber broken;

[0021]FIG. 6 is a schematic block diagram of a prior art circuit forswitching to a spare fiber in an optical transmission system;

[0022]FIG. 7a is a schematic block diagram of a circuit for switching toa standby transmitter in an optical transmission system in which alltransmitters are operating correctly;

[0023]FIG. 7b is a schematic block diagram of a circuit for switching toa standby transmitter in an optical transmission system in which onetransmitter is faulty;

[0024]FIG. 8a shows an example of the use of the optical switching unitaccording to the invention as an optical selector switch in an expansionunit of an optical switching network; and

[0025]FIG. 8b shows an example of the use of the optical switching unitaccording to the invention as an optical selector switch in adistribution unit of an optical switching network.

[0026]FIG. 1 shows an optical switching unit according to the invention.The switching unit OSE contains an optical space switch ORS which has nports P1A . . . PNA on the left-hand side and m ports P1B . . . PMB onthe right-hand side. At each port, light can enter or leave or bothenter and leave the optical space switch ORS depending on whetherunidirectional or bidirectional transmission is taking place. Theoptical space switch enables each of the left-hand ports P1A . . . PNAto be connected to every right-hand port P1B . . . PMB. In mostapplications, n will be chosen equal to m, i.e., the space switch willhave equal numbers of ports on both sides. Applications are alsoconceivable, however, in which n is not equal to m. This variant isdealt with in an embodiment explained below.

[0027] In FIG. 1, the port P1A on the left-hand side of the space switchORS is monitored by a monitoring unit M1A. On the right-hand side of thespace switch ORS, a monitoring unit M1B monitors the port P1B. Themonitoring units are preferably photodiodes, but it is also possible touse other components, such as photomultipliers. “To monitor the ports”as used herein means that the monitoring units M1A and M1B at leastdetermine whether light is entering the optical space switch ORS at therespective port or not. In a broader sense, “to monitor” may also meanthat the monitoring units M1A and M1B are capable of checking signalsapplied at the respective port for their information content. In thatcase, these monitoring units also include demultiplexing and/or decodingdevices, for example, depending on the transmission technique used. Theinformation thus obtained is transferred from the monitoring units M1Aand M1B over links V1A and V1B to a control unit SE. Based on thisinformation, the control unit then controls the position of the opticalspace switch ORS via the optical signals at the ports. The control unitpreferably comprises a microprocessor and a memory unit. Therelationship between the switch position and the signals applied at theports being monitored is determined by programming or by hardwired logicand depends on the application of the optical switching unit.

[0028]FIG. 2 shows a specific embodiment of the optical switching unitaccording to the invention. Here the optical space switch ORS is a 2×2space switch as is currently obtainable as an integrated opticalcomponent. The space switch ORS is connected to optical waveguides,e.g., optical fibers or integrated optical waveguides. The ports of theoptical space switch ORS are designated P1A, P2A and P1B, P2B. Connectedahead of the port P1A is an optical directional coupler K1A. Thedirectional coupler K1A, for example a fused fiber coupler, taps off aportion of the optical signal applied at the port P1A and feeds it tothe photodiode PD1A. The photodiode is connected to the control unit SE.In analogous fashion, the other three ports are monitored by photodiodesPD2A, PD1B, PD2B. The control unit comprises a microprocessor andcontrols the position of the optical space switch.

[0029]FIG. 3 shows a particularly advantageous embodiment. Unlike theembodiment illustrated in FIG. 2, this embodiment requires only twodirectional couplers K1 and K2 to tap off light from all four ports P1A,P2A, P1B, P2B. As in the above embodiment, the port P1A is monitored bythe photodiode PD1A. The port P1B on the other side of the optical spaceswitch ORS, however, is monitored by the photodiode PD3 or PD4,depending on the position of the optical space switch ORS. If theoptical space switch ORS is set at through-connection (continuous linein FIG. 3), the signal applied at port P1B will pass through the opticalspace switch and the directional coupler K1 to the photodiode PD3. Ifthe optical space switch ORS is set at cross connection (dashed line),the signal applied at port P1B will pass through the optical spaceswitch and the directional coupler K2 to the photodiode PD4. Since thecontrol unit SE knows the position of the optical space switch ORS, itcan unambiguously assign the information provided by the fourphotodiodes PD1A, PD2A, PD3, PD4 to the individual ports. Since thisembodiment requires only two directional couplers, the optical switchingunit can be of a very compact design.

[0030] If the optical space switch ORS is an n×m space switch with n≠m,the directional couplers must be located on that side of the opticalspace switch which has the greater number of ports. In a 4×8 spaceswitch, for example, this eliminates the need for four directionalcouplers.

[0031] In the embodiment shown in FIGS. 4a and 4 b, the opticalswitching unit according to the invention is used to switch from anoptical fiber F to a spare fiber EF in a bidirectional opticaltransmission system. In FIG. 4a, the path between the communication endpoints PA and PB is divided into four sections by three opticalswitching units OSE1, OSE2, and OSE3 according to the invention. Theoptical switching units are set at through-connection and thus connectthe communication end point PA to the communication end point PB via theoptical fiber F.

[0032] If, as sketched in FIG. 4b, an interruption in transmissionoccurs between the two optical switching units OSE2 and OSE3, forexample because the fiber has broken or has been bent at too small aradius, the optical switching units OSE2 and OSE3 adjacent to the breakBS will detect the absence of input signals on the respective sidesfacing the break BS. They respond to this immediately by switching tothe spare fiber EF. Thus, changeover to the spare fiber is effected onlyin the faulty section of the transmission path. If the spare fiber EFshould be damaged in the section between the optical switching unitsOSE1 and OSE2, this division of the transmission path according to theinvention still permits undisturbed data transmission.

[0033] The principal advantage of this solution over prior art solutionsis that the switching actions are initiated at decentralized locations,i.e., by the switching units themselves and not by a central controlstation associated with the end points of the transmission path. As aresult, very short dead times are possible; in addition, thetransmission path can be modular in construction and be supplementedwithout the need to make any changes at the communication end points.

[0034] In the embodiment illustrated in FIG. 5, the principle shown inFIG. 4 for bidirectional transmission is adapted to unidirectionaltransmission. The unidirectional transmission takes place from thecommunication end point PA to the communication end point PB. If a breakBS occurs in the section between the optical switching units OSE2 andOSE3, the optical switching unit OSE3 will detect no signal at theassociated port and then switch to the spare fiber EF. To also enablethe optical switching unit OSE2, located to the left of the break inFIG. 5, to detect the disconinuity in the optical fiber F, an opticalcheck signal generated by a check-signal transmitter KS is coupled intothe optical fiber F from the receiver end PB. The check-signaltransmitter may be a semiconductor laser, for example; coupling to theoptical fiber F is preferably effected via a fused fiber coupler VK.

[0035] This signal, which is injected in the direction of arrow P,passes through the optical fiber F in a direction opposite to thedirection of transmission. Since after occurrence of the break theoptical switching unit OSE3 has already switched (see continuous linesin OSE3), the check signal passes along the spare fiber EF and thusreaches the associated port of the optical switching unit OSE2. Withouta break in the optical fiber F, the check signal would have been routedto the adjacent port, i.e., the upper port in FIG. 5. Since, in theevent of a break in the optical fiber F, the check signal isautomatically routed to the other port of the optical switching unitOSE2, this optical switching unit OSE2 detects the break immediately andswitches to the spare fiber EF without delay, so that the signal to betransmitted now passes along the spare fiber EF.

[0036] Another embodiment of the present invention is shown in FIGS. 7aand 7 b. In this embodiment, the optical switching unit according to theinvention is used in a standby arrangement for optical transmitters. Inthe undisturbed condition, each of four transmitters S1 . . . S4 isconnected to a respective one of four optical fibers F1 . . . F4 viarespective optical switching units OSE1 . . . OSE4. The states of theoptical switching units are indicated in FIG. 7a by dashed lines. Inthis normal state, a check-signal transmitter KS is connected to astandby transmitting unit RES. The standby transmitting unit comprisesan optical transmitter which is preferably implemented in the samemanner as the four transmitters S1. . . S4. In addition, the standbytransmitting unit incorporates a photodiode which monitors the presenceof a signal. When none of the four transmitters S1 . . . S4 is faulty,this photodiode detects the check signal emitted by the check-signaltransmitter KS. As long as this signal is detected, the opticaltransmitter in the standby transmitting unit RES remains in the standbymode, i.e., it does not emit light.

[0037] If, for example, the transmitter S3 fails, as shown in FIG. 7b,the optical switching unit OSE3 will detect the absence of a signal atthe associated port. The optical switching unit will then change itsstate. The check-signal transmitter KS is now connected to the failedtransmitter S3 rather than the standby transmitting unit RES. The twooptical switching units OSE2 and OSE1 then detect the absence of thesignal emitted by the check-signal transmitter KS and change theirstates as well. Since the standby transmitting unit RES is no longerreceiving a check signal, it will change from the standby mode to theactive mode and emit light. Thus, the end state shown in FIG. 7b hasbeen reached. The standby transmitting unit RES and the transmitters S1,S2, and S4 are now connected to the optical fibers F . . . F4. After thefailure of the transmitter S3, the connection is thus maintainedvirtually unchanged.

[0038] Depending on whether all four optical fibers are fed withidentical or different signals, electronics of suitable design mustensure that upon changeover to the standby transmitting unit RES, themodulation currents supplied to the optical transmitters S1. . . S4 arediverted appropriately. Furthermore, it may be advantageous to selectfor the signal emitted by the check-signal transmitter KS a separatewavelength which is different from the transmission wavelength. In thismanner, light which does not originate from the check-signal transmitterKS is effectively prevented from being coupled into the standbytransmitting unit RES by crosstalk in the optical switching units.

[0039] Here, too, the essential advantage of the use of the opticalswitching unit according to the invention is that switching of the spaceswitches is effected without the need for a central control unit and,therefore, without delay.

[0040] In a further embodiment, the optical switching unit is used as anoptical selector switch. Such optical selector switches are needed forall-optical switching in optical transmission systems. In an opticalexchange, like in electronic exchanges, switching networks are to beused for concentrating, distributing, and expanding transmission lines.In packet-switched optical transmission systems, the packet headersinclude data fields for the address. This address must be recognized bythe switching network so that the call can be guided to the correctdestination.

[0041]FIG. 8a shows a simple example of an expansion unit in an opticalswitching network. By the use of three optical switching units accordingto the invention, OSE0, OSE11, and OSE12, the input E can reach any ofthe four outputs A1 . . . A4. At the port of the optical switching unitOSE0, the data packets are monitored by a photodiode. The control unitincorporated in the optical switching unit according to the inventionrecognizes the address in the data packet and, depending on how theswitching unit is programmed, causes the signals to be directed to oneof the two outputs of the optical switching unit. The two opticalswitching units OSE11 and OSE12 in the second switching-network planeoperate correspondingly. Thus, in this embodiment, optical spaceswitches as are commonly used in switching networks are not controlledby a central control unit connected to all space switches, but theycontrol themselves, so to speak. As a result, a modular construction ofsuch switching networks becomes much simpler and less costly. It shouldbe noted that in this embodiment, the numbers of ports on the two sidesof the optical space switch are not equal (n≠m).

[0042]FIG. 8b shows a distribution unit in a switching network. Each ofthe four ports A1 . . . A4 on the left-hand side of the distributionunit is connectable via the five optical switching units OSE1 . . . OSE5to any one of the four ports B1 . . . B4 on the right-hand side. Eachoptical switching unit incorporates a 2×2 optical space switch whosefour ports are monitored by one photodiode each. This makes it possibleto process call requests arriving from both sides of the distributionunit.

[0043] While only a few simple examples of the possible applications ofthe optical switching unit according to the invention have beendescribed, it is readily apparent from these examples that the opticalswitching unit according to the invention has many applications inoptical transmission and switching systems.

1. An optical switching unit for use in optical transmission systems,comprising an n×m optical space switch (ORS) having n+m ports (P1A . . .PNA; P1B . . . PMB), with n=1, 2, 3, 4, . . . and m=2, 3, 4, . . . ,characterized in that means (k1a, k1b, m1a, m1b, se) are provided formonitoring the optical signals arriving at at least one port (p1a, p1b)on each side of the optical space switch (ors) and for controlling theposition of the n×m space switch (ors) in accordance with changes in theoptical signals being monitored:
 2. An optical switching unit as claimedin claim 1 wherein the means monitor all signals entering through theports (P1A, P2A in FIG. 3) on one side of the optical space switch (ORS)and all signals exiting through said ports.
 3. An optical switching unitas claimed in either of the preceding claims wherein the means comprisephotodiodes (PD1A, PD2A, PD1B, PD2B in FIG. 2), directional couplers(K1A, K1B, K2A, K2B) and a control unit (SE).
 4. An optical transmissionsystem comprising a transmission facility, at least one standbytransmission facility, and at least two optical switching units asclaimed in any one of the preceding claims wherein, if the transmissionfacility does not operate correctly, the at least two optical switchingunits detect this and switch autonomously to one of the standbytransmission facilities.
 5. A bidirectional optical transmission systemcomprising: a) a first transceiver unit; b) a second transceiver unit;c) a first optical waveguide (F in FIGS. 4a and 4 b) connecting thefirst transceiver unit with the second transceiver unit; d) at least kfurther optical waveguides (EF) arranged parallel to the first opticalwaveguide (F), with k=1, 2, 3, . . . ; and e) at least two opticalswitching units (OSE1, OSE2, OSE3) as claimed in any one of claims 1 to3, which are integrated into the optical waveguides (F, EF), and which,if any one of the total of k+1 optical waveguides (F, EF) is damaged inat least one of the sections located between two optical switchingunits, switch autonomously to another optical waveguide in said section.6. A unidirectional optical transmission system comprising: a) a firsttransmitter; b) a receiver; c) a first optical waveguide (F in FIG. 5)which connects the first transmitter with the receiver and in which thesignals to be transmitted propagate in a first direction of propagation;d) at least k further optical waveguides (EF) arranged parallel to thefirst optical waveguide (F), with k=1, 2, 3, . . . ; e) a secondtransmitter (KS); f) a coupler (VK) which couples the light generated bythe second transmitter (KS) into the first optical waveguide such thatit propagates in a direction opposite to the first direction ofpropagation; and g) at least two optical switching units (OSE2, OSE3 inFIG. 5) as claimed in any one of claims 1 to 3 which are incorporated inthe waveguides and which, if any one of the total of k=1 opticalwaveguides (F, EF) is damaged in at least one of the sections locatedbetween two optical switching units, switch to another optical waveguidein said section.
 7. An optical transmitting unit comprising: a) a firsttransmitter; b) an optical switching unit having 2×2 ports as claimed inany one of claims 1 to 3 which, if the first transmitter operatesproperly, optically connects the latter to an optical waveguide; c) astandby transmitting unit comprising a standby transmitter and areceiver; and d) a second transmitter which is so connected to theoptical switching unit that, if the first transmitter operates properly,the second transmitter is optically connected to the receiver of thestandby transmitting unit, and that, if the first transmitter is faulty,and the optical switching unit detects this and operates the opticalspace switch, the second transmitter is not optically connected to thestandby unit, but the standby transmitter of the standby transmittingunit is optically connected to the optical waveguide.
 8. An opticaltransmitting unit comprising: a) n transmitters (S1 . . . S4), with n=2,3, 4, . . . ; b) n optical waveguides (F1 . . . F4); c) n opticalswitching units (OSE1 . . . OSE4) having 2×2 ports as claimed in any oneof claims 1 to 3 which, if all of the n transmitters (S1 . . . S4)operate properly, each connect a respective one of the n transmitters(S1 . . . S4) optically to a respective one of the n optical waveguides(F1 . . . F4), and which are interconnected by respective opticalwaveguides; d) a standby transmitting unit (RES) comprising a standbytransmitter and a receiver; and e) an additional transmitter (KS inFIGS. 7a/7 b) which is so connected to one of the n optical switchingunits (OSE1 . . . OSE4) that, if all of the n transmitters (S1 . . . S4)operate properly, the additional transmitter (KS) is optically connectedthrough all of the optical switching units (OSE1 . . . OSE4) to thereceiver of the standby transmitting unit (RES), and, if one (S3) of then transmitters (S1 . . . S4) is faulty, and the optical switching unit(OSE3) connected to said optical transmitter detects this and operatesthe optical space switch, the additional transmitter (KS) is notconnected to the standby transmitting unit (RES), but the standbytransmitter of the standby transmitting unit (RES) is opticallyconnected to that optical waveguide which in the undisturbed conditionwas connected to the transmitter which is now faulty.
 9. An opticalswitching network comprising a plurality of selector switches,characterized in that the selector switches are optical switching unitsas claimed in any one of claims 1 to 3.